The present invention relates to an objective lens for an optical pickup apparatus and the optical pickup apparatus, and particularly, to the objective lens for the optical pickup apparatus, and the optical pickup apparatus which has the good temperature characteristic.
Recently, a research and development of the high density optical system by which the recording and/or reproducing (hereinafter, “recording and/or reproducing” is written as “recording/reproducing”) of the information can be conducted, by using the blue violet semiconductor laser of the oscillation wavelength of about 400 nm, is quickly advanced. As an example, in an optical disk by which the information recording/reproducing is conducted by a specification of NA 0.85, light source wavelength 405 nm, so called Blue-ray Disk (hereinafter, BD is used), to the optical disk of the diameter 12 cm whose size is the same as DVD (NA 0.6, light source wavelength 650 nm, the recording capacity is 4.75 GB), the information of 23-27 GB per 1 layer can be recorded, further, in an optical disk by which the information recording/reproducing is conducted by a specification of NA 0.65, light source wavelength 405 nm, so called HD DVD (hereinafter, called HD), to the optical disk of the diameter 12 cm, the information of 15-20 GB per 1 layer can be recorded. Hereinafter, in the present specification, such an optical disk is called as “high density optical disk”.
Hereupon, as a performance required for the objective lens for the optical pickup apparatus, there are the “temperature characteristic” and “wavelength characteristic”. The temperature characteristic is expressed by a change amount of the spherical aberration to the change of the environmental temperature, and the wavelength characteristic is expressed by a change amount of the spherical aberration to the oscillation wavelength change of the semiconductor laser, and as small as an extent of values of the both is, it is preferable as the objective lens for the optical pickup apparatus. However, for the time of the recording/reproducing of the information to the high density optical disk, the compatibility of the temperature characteristic and the wavelength characteristic due to the deviation of the oscillation wavelength of the blue violet semiconductor laser is difficult.
To such a problem, in Tokkai (Japanese Unexamined Patent Publication) No. 2002-82280, the light source, and the light-converging optical system including the objective lens for light-converging the light flux emitted from the light source through the transparent substrate of the optical information recording medium on the information recording surface, and the optical pickup apparatus having the optical detector for light-receiving the reflected light from the optical information recording medium, the optical pickup apparatus characterized in that a means for correcting a fine variation of the oscillation wavelength of the light source and a variation of the spherical aberration generated in the light-converging optical system due to the temperature and the humidity change is provided, is disclosed.
However, in Tokkai No. 2002-82280, as the correcting means for correcting the variation of the spherical aberration, a beam expander of 2-lens composition by which the lens interval can be changed, is shown. By using this engineering, in order to correspond to the fluctuation of the oscillation wavelength, it is necessary to control the correcting means corresponding to the fluctuation of the oscillation wavelength among respective semiconductor lasers (individual difference of semiconductor laser), for example, it is necessary to provide a setting means for setting a lens position as the initial position after adjusting the lens position of the beam expander being a correction means for each oscillation wavelength of a semiconductor laser to be incorporated in an optical pickup apparatus, there is a problem that the structure or assembly of the optical pickup apparatus becomes complex.
On the one hand, in Tokkai No. 2004-177527, when a correction element having a fine step structure is arranged before the glass made objective lens, an engineering by which the spherical aberration of the optical system to the wavelength change of the light source is corrected, is shown. However, when the glass made objective lens is replaced with a plastic made objective lens for the purpose of the weight reduction or others, a new problem is generated. That is, because a general plastic has a large temperature dependency of the refractive index, the spherical aberration change due to the temperature change is apt to become large. As a means for reducing this, a fine step difference structure is provided, a means for suppressing the spherical aberration change by using the change of the oscillation wavelength of the semiconductor laser following the temperature change is well known, however, when the spherical aberration change due to the wavelength change is given so that the spherical aberration change following the temperature change is cancelled, there is a problem that the spherical aberration change due to the wavelength change so as to correct the fluctuation of the oscillation wavelength of respective semiconductor lasers can not be given and it is difficult to counter the deviation of an oscillation wavelength due to individual difference of semiconductor lasers.